1. Field of the Invention
The present invention relates to water or waste water filters using underdrains and, in particular, to a method of distributing gas and liquid through such underdrains.
2. Description of Related Art
Gravity filters are used to separate suspended solids from water. The main components of gravity filters are filtering media and underdrain blocks. Underdrain blocks are generally square, rectangular, or triangular in cross section and have a long longitudinal axis as compared to their cross section. They are attached end-to-end to form long sections called laterals, or are extruded to length, or are fabricated to length. Filter underdrain laterals are laid on the floor of the filter tank one next to the other in parallel rows to define gas and liquid flow conduits below a bed of filtering media. The filtering media is used to capture dirt or contaminant particles from a liquid being filtered as the liquid passes through the media. The filter media is supported by a media retainer so that it will not pass into the underdrain block or collection system.
The gas and liquid flow conduits make possible the collection of filtered liquid during filtration and the distribution of gas and liquid for backwash. Typically, a common main conduit (known as a “flume”) is located immediately next to or under the filter tank to collect the filtered liquid from the underdrain laterals during filtration and to distribute gas and/or liquid to the underdrain laterals during gas (typically air) or liquid (typically water) backwash. The common main conduit also distributes gas and liquid separately to each of the underdrain laterals during backwash. Oftentimes, to prepare the granular media for water backwash, the submerged granular media is first scoured with a gas-only backwash, where gas is pumped upward through underdrain block. The gas-only backwash loosens the dirt and other impurities so that they will be more easily carried off by a subsequent liquid and/or concurrent gas and liquid backwash phase.
It is sometimes desired to scour the filter media at a lower than customary gas rate, while also being able to distribute gas at traditional rates during other steps and processes. However, lateral-style filter underdrains do not typically distribute air sufficiently evenly across the area of a filter at these low rates. A need, therefore, exists for a method of distributing gas at a low scour rate evenly and efficiently across the area of a filter system while also still being able to distribute gas at traditional rates.